Project Summary/Abstract: Project 4 (University of Pittsburgh) Defining mechanisms that underlie brain resilience to Alzheimer?s disease (AD), the discordance between normal cognition and substantial AD pathology, which occurs in preclinical AD, will provide insights into novel therapeutic targets to prevent dementia. To address this concept, we propose to investigate alterations in neurotransmitter systems and synaptic proteins within the default mode network (DMN), a cortical connectome involving the precuneus (PreC), posterior cingulate cortex (PCC), and frontal cortex (FC), which is prone to amyloid pathology and functional connectivity changes in preclinical and prodromal [mild cognitive impairment, (MCI)]. Project 4 demonstrated a loss of synapses within DMN regions in early AD, but not in MCI compared to subject with no cognitive impairment (NCI). In contrast, we observed upregulated cholinergic activity in FC, but not in the PreC compared to glutamatergic denervation in both regions in MCI. This suggests that a DMN region-specific cholinergic neuroplasticity response, may contribute to brain resilience. Whether similar mechanism(s) occur in preclinical AD is of key importance for drug target discovery. Neuropsychological and neuropathological evaluation of Rush Religious Order Study subjects provides a unique opportunity to investigate compensatory synaptic and neurotransmitter responses associated with resilience in conjunction with AD pathology in NCI with low (LP-NCI) or high (HP-NCI) pathology, MCI and AD. Aim 1A will use confocal immunofluorescence microscopy with unbiased stereology to determine whether different neurochemically- defined synaptic terminals within the DMN conectome are resilient or vulnerable in HP-NCI, MCI, and AD relative to LP-NC. We will correlate these findings with cognitive test scores across groups. Aim 1B will use a novel liquid chromatography-tandem mass spectrometry (LC-MS/MS) targeted proteomic assay to determine whether different synaptic proteins within the DMN interactome are resilient or vulnerable in same cases. Aim 2 will determine whether AD neuropathology in the DMN interactome moderates the relationship between synaptic and cognitive measures, by quantifying Tau and amyloid concentrations and pathology in the DMN in the same Aim 1 cases. Aim 3 will examine whether markers of neuroinflammation moderate the relationship between synaptic loss and cognition in the DMN using the same Aim 1 and Aim 2 cases. The translational value of Project 4 is highlighted by our use of in vitro binding assays with tritiated ligands for Tau and amyloid PET imaging, in the same brain regions analyzed for quantitative biochemistry of Tau, amyloid and synaptic markers. The proposed studies will provide pivotal data needed to understand the mechanisms underlying circuit-specific resilience or vulnerability to AD pathology in the preclinical stage of the disease and provide insight into potential new drug therapies.